1. Field of the Invention
The present invention relates to vertical refuse incinerators for incinerating wastes having a wide variety of properties, in particular, industrial wastes including medical wastes, and to methods for controlling the same.
2. Related Art
Industrial wastes contain not only many hazardous materials, but also materials with a high heating value and hard-to burn materials or incombustible materials. In addition, industrial wastes occurs in a wide variety of shapes, such as solid, liquid and viscous, so that it has been extremely difficult to completely dispose of such industrial wastes with conventionally used fixed grate batch type incinerators.
For incineration of medical wastes having a wide variety of properties and including hazardous infectious materials containing pathogenic viruses and easily meltable materials such as glass, for example, rotary kiln type incinerators, inclined rotary hearth type incinerators, horizontal rotary hearth type incinerators equipped with agitating means are commonly used. Since each of these incinerators uses a method in which wastes are burnt while being turned and agitated, this causes uneven combustion, or only flammable materials to be burnt first to result in a burnout of the grate portion, and hard-to burn materials remain unburnt. This has made it impossible to perform the complete combustion and sterilization of wastes, leading to the problem of not being able to prevent, in particular, the generation of dioxins due to incomplete combustion and the discharge of unburnt materials. The method in which refuse is incinerated while being agitated has also caused such deficiencies as an increased generation of dioxins due to the catalysis of fly ash generated in large amounts. Furthermore, there has been the problem that glasses are melted and attached to the outlet portion of the incinerators, thus making it impossible to continue the operation.
In the case of incinerating gereral wastes having a wide variety of properties, there have been also problems similar to those described above, such as the burnout of the grate portion, incomplete combustion and the generation of dioxins.
FIG. 10 is a vertical sectional view schematically showing the “Vertical incinerator and incineration method thereof” disclosed in Japanese Laid-Open Patent Publication No. Hei 4-158110, which is related art that solves the above-described problems.
Referring to FIG. 10, a combustion gas exhaust port 206 is mounted at the top of an incinerator body 201, and a hopper 202 having a feeder and an ignition burner 203 are provided at the upper portion. Inside the incinerator body 201, retractable refuse supporting plates 204 are provided at the lower portion, and closable bottom ash discharge plates 205 are arranged at the bottom.
As shown in FIG. 10, the refuse supporting plates 204 are usually arranged in positions that are retracted from the inside of the incinerator body 201, and are projected into an upper portion of an ash layer AL so as to support the load of refuse and bottom ash located above the refuse supporting plates 204 only when the bottom ash discharge plates 206 are opened so as to discharge bottom ash, as indicated by the dash-dotted line in FIG. 10.
On both sides of the incinerator body 201 where the refuse supporting plates 204 are located, compartments 210 are provided for housing the refuse supporting plates 204 when the refuse supporting plates 204 are retracted from the inside of the incinerator body 201.
A room-temperature cooling air stream CA is supplied to the compartments 210, and the cooling air stream CA is jetted into the incinerator body 201 from clearances 211 formed between the incinerator body 201 and the compartments 210, cooling the refuse supporting plates 204, while preventing bottom ash in the incinerator body 201 from entering into the compartments 210 from the clearances 211.
The bottom ash discharge plates 206 are closably provided at the bottom of the incinerator body 201 such that they can be opened and closed between a horizontal position and the vertical position indicated by the dash-dotted line. By turning the bottom ash discharge plates 205 downward after supporting the layers located above the upper portion of the ash layer AL in the lower portion of the incinerator body 201 with the refuse supporting plates 204, the incinerated bottom ash BA can be carried out to an ash removal conveyor 212 provided below the incinerator body 201.
That is to say, the refuse supporting plates 204 are provided to assist the bottom ash discharge plates 205 in discharging the bottom ash BA.
In addition, combustion air streams A1, A2 and A3 whose temperatures are adjusted are supplied via dampers 221, 222 and 223 to the upper, middle and lower portions of the incinerator body 201, respectively. The temperature of each of the combustion air streams A1, A2 and A3 is adjusted to an optimal temperature in accordance with the property of the refuse.
The ignition burner 203 mounted on the side of the incinerator body 201 that is opposite from the side where the hopper 202 is provided is used to ignite refuse at the time of start-up or to aid combustion when the temperature inside the incinerator is low.
Next, a method for incinerating refuse with a vertical incinerator having the above-described structure is described.
Here, in the incinerator body 201 at normal operation, a flame zone FZ, a refuse layer RL, a glow layer GL and an ash layer AL are formed from top to bottom in this order. The positions of these layers move, depending on the combustion state of refuse rising successively from the lower layer.
Refuse supplied from the hopper 202 into the incinerator body 201 is deposited on the ash layer AL located at the bottom of the incinerator body 201 at the period of start-up, and heated by the ignition burner 203 and its combustion is started with the combustion air streams A1 and A2. Then, flammable refuse is incinerated to ash first and deposited in the glow layer GL along with hard-to burn refuse, while retaining the embers.
If refuse is supplied in this state, the refuse is deposited in the refuse layer RL, and the flammable materials start to ignite first with the heat of the glow layer GL and the combustion air stream A1. Then, the combustion gradually extends throughout the refuse layer RL, shifting the operation to its normal state.
During this combustion, a combustion gas stream CG generated in the glow layer GL and a lower portion of the refuse layer RL passes through the refuse layer RL and rises, promoting the ignition and gasification of refuse located thereabove and drying garbage with its heat.
Further, the combustion gas stream CG that has risen to the flame zone FZ is reburnt with a room-temperature secondary air stream SA supplied thereabove, and then discharged as exhaust gas from the combustion gas exhaust port 206 for the next step.
The radiation heat generated during this re-combustion of the combustion gas stream CG in the flame zone FZ is used to perform a preliminary drying of refuse charged into the refuse layer RL and to burn paper or plastic, each having a low ignition point, promoting these materials to become the embers.
After completion of the combustion in the ash layer AL, the refuse supporting plates 204 are projected into the upper portion of the ash layer AL in the incinerator body 201 so as to support the load of the bottom ash BA and refuse in the refuse layer RL, the glow layer GL and the upper portion of the ash layer AL that are located above the refuse supporting plates 204.
At the time of this projection, the combustion of refuse has been completed in the positions where the refuse supporting plates 204 are located, so that the refuse supporting plates 204 can be smoothly projected, with little resistance due to the refuse.
After projecting the refuse supporting plates 204 in this manner, the bottom ash discharge plates 205 are turned downward so as to drop the bottom ash BA in a discharge area DA that is located below the refuse supporting plates 204, into the ash removal conveyor 212.
After discharging the bottom ash BA, the bottom ash discharge plates 205 are turned upward to be restored, and then the refuse supporting plates 204 are retracted from the inside of the incinerator body 201 into the compartments 210 so as to drop the remaining bottom ash BA located above the refuse supporting plates 204 and the incineration residue in the glow layer GL, onto the bottom ash discharge plates 205 at the bottom, while also successively dropping the refuse layer RL.
The shock generated during the dropping not only improves the air permeability of the ash layer AL, but also breaks up lump of unburnt materials in the glow layer GL and the refuse layer RL, which improves the air permeability of the layers and allows air to pass through the inside of the lump. Accordingly, when the high-temperature combustion air streams A2 and A3 are supplied, the unburnt materials in the bottom ash BA can be readily burnt with the retained embers.
However, it is difficult to perform the complete combustion and sterilization of industrial wastes, in particular, medical wastes, with the conventional vertical incinerator. The reason is that such wastes contain materials with a high heating value and hard-to burn materials or incombustible materials and have a variety of properties, causing violent fluctuation in the temperature inside the incinerator and thus resulting in unstable combustion.
Additionally, the secondary combustion in the flame zone FZ is not performed completely in the vertical incinerator shown in FIG. 10, so that the thermal decomposition of dioxins is insufficient in the incinerator. This not only may necessitate an increase in the capacity of the incinerator body 201 and that of a subsequent re-combustion chamber (not shown), but also may place an extra burden on subsequent exhaust gas treatment equipment (also not shown).
Moreover, glasses such as syringes, test tubes and medicine bottles that are contained in large amount in wastes are softened and melted at 400 to 700° C., the calcium content contained in various construction materials or plaster casts is softened and melted at 850° C. or higher, and the ash content is melted due to high heat generated by partial combustion of materials with a high heating value including for example plastics such as expanded polystyrene, paper and fibers, thereby often forming solid clinkers.
This has posed the following problems: A blockage situation due to the clinker may occur in the vicinity of the glow layer GL in the lower portion of the incinerator body 201, which impedes the fall of the refuse or the bottom ash BA in the upper portion, leading to a suspension of the operation in order to take away the clinkers. In the case of using a simple single plate structure or refuse supporting plates 204 having no forced cooling means in which a plurality of, for example, comb-shaped supporting rods are provided, the above-described clinkers impede the projection of the refuse supporting plates 204 and may cause damage to the refuse supporting plates 204 in the worst-case scenario.
In addition, when the vertical incinerator is increased in capacity, its strength becomes insufficient due to the cantilever structure of the refuse supporting plates 204 and the refuse supporting plates 204 may be broken and damaged in the case where the clinkers are generated.
Furthermore, at the time of dropping the ash in the lower portion onto the bottom ash discharge plates 205, the thickness of the ash layer AL becomes thin when the amount of incombustible components is small, so that a part of the glow layer GL may be dropped and burnt in the discharge area DA. Or, when unburnt material remains, the unburnt material is broken up by the shock generated during the dropping and similarly burnt in the discharge area DA, so that the clinkers may be generated in the vicinity of the ash layer AL, causing damage to the refuse supporting plates 204 that are projected during the discharge of the bottom ash BA.
On the other hand, since the bottom of the incinerator is completely cooled after the incinerator is out of operation for a long time for a repair work or periodic maintenance work, it requires a long time to increase the temperature in the incinerator from restart to normal operation.